Borrowings and Credit Arrangements Principle Maturities of Borrowings and Credit Arrangements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Credit Facilities [Line Items]
2019	$ 21,423
2020	28,691
2021	28,000
2022	74,000
2023	0
Thereafter	150,000
Total maturities of borrowings and credit arrangements	302,114
Revolving Credit Facility
Credit Facilities [Line Items]
2019	20,000
2020	28,000
2021	28,000
2022	74,000
2023	0
Thereafter	0
Total maturities of borrowings and credit arrangements	150,000
Convertible Notes Payable | 3.50% Convertible Senior Notes due October 1, 2024
Credit Facilities [Line Items]
2019	0
2020	0
2021	0
2022	0
2023	0
Thereafter	150,000
Total maturities of borrowings and credit arrangements	150,000
Structured Payables
Credit Facilities [Line Items]
2019	1,423
2020	691
2021	0
2022	0
2023	0
Thereafter	0
Total maturities of borrowings and credit arrangements	$ 2,114	$ 709